1. Field of the Invention
This invention relates to a semiconductor sensor having a movable portion of a beam structure for detecting a physical quantity such as acceleration, yaw rate, vibration, or the like.
2. Description of the Related Art
A differential capacitance type semiconductor sensor detects a physical quantity such as acceleration based on a change in capacitance between a movable electrode and a fixed electrode. A conventional method for manufacturing the sensor has a problem that, in a dry step which is carried out after the movable electrode and the fixed electrode are formed, the movable electrode is liable to be attached to the fixed electrode facing thereto due to surface tension of demineralized water intervening therebetween, and a beam is liable to be attached to a substrate facing thereto due to the same reason. In addition, when the sensor detects the acceleration, if acceleration is produced with a magnitude larger than a specific value that is determined by a spring constant of the beam, the movable electrode is attracted to the fixed electrode. The attracted movable electrode is attached to the fixed electrode by a van der Waals force even after the acceleration is decreased.
To solve the problem, U.S. Pat. No. 5,542,295 proposes a sensor. As shown in FIG. 1, the sensor has a stopper portion 203 protruding from a beam portion 201 toward a weight portion 202 so that a gap between the beam portion 201 and the weight portion 202 is smaller than that between a movable electrode and a fixed electrode. Accordingly, the movable electrode is prevented from being attached to the fixed electrode.
However, as indicated by a dotted line in FIG. 2, the stopper portion 203 readily deforms by displacement of the beam portion 201 so that the gap between the beam portion 201 and the weight portion 202 is largely increased. Accordingly, the movable electrode can move more than the gap between the movable electrode and the fixed electrode such that it is attached to the fixed electrode. This kind of problem occurs prominently in the differential capacitance type physical semiconductor sensor in which the gap between the movable electrode and the fixed electrode is very small, and especially when the beam portion 201 is displaced not only in a substrate surface direction but also in a direction perpendicular to the substrate surface direction as in a resonant state.
On the other hand, JP-A-6-347474 proposes a sensor in which one of a movable electrode and a fixed electrode has a protrusion as a stopper so that a contact area between the movable electrode and the fixed electrode decreases and so that the movable electrode is prevented from being attached to the fixed electrode. However, when the protrusion is directly formed with the movable electrode or the fixed electrode, a capacitance between the movable electrode and the fixed electrode can readily change so that a change in capacitance cannot be precisely detected. Further, when a potential difference between the movable electrode and the fixed electrode is produced, the movable electrode can be attached to the fixed electrode by an electrostatic force produced therebetween.
Further, JP-A-9-211022 proposes an acceleration sensor in which a substrate has a protrusion facing a beam portion for preventing the beam portion from being attached to the substrate. However, there is a case that not the beam portion but a movable electrode is attached to the substrate by an electrostatic force produced between the movable electrode and the substrate. Therefore, it is not sufficient that the substrate has the protrusion only at the lower portion of the the beam portion.